Chapter 6: Cell Growth and Reproduction Lesson 6.2: Chromosomes and DNA Replication Cell reproduction involves a series of steps that always begin with the processes of interphase. During interphase the cell’s genetic information which is stored in its nucleus in the form of chromatin, composed of both mitotic and interphase chromosomes molecules of protein complexes and DNA strands that are loosely coiled winds tightly to be replicated. It is estimated that the DNA in human cells consists of approximately three billion nucleotides. If a DNA molecule was stretched out it would measure over 20 miles in length and all of it is stored in the microscopic nuclei of human cells. This lesson will help you to understand how such an enormous amount of DNA is coiled and packed in a complicated yet organized manner. During cell reproduction as a cell gets ready to divide the DNA coils even more into tightly compact structures. Lesson Objectives • Describe the coiled structure of chromosomes. • Understand that chromosomes are coiled structures made of DNA and proteins. They form after DNA replicates and are the form in which the genetic material goes through cell division. • Discover that DNA replication is semi-conservative; half of the parent DNA molecule is conserved in each of the two daughter DNA molecules. • Outline discoveries that led to knowledge of DNA’s structure and function. • Examine the processes of DNA replication. Vocabulary • centromere • double helix • Chargaff’s rules • histones • chromatid • nucleosomes • chromatin • semi-conservative DNA replication • chromosome • sister chromatids • DNA replication • transformation Introduction In eukaryotic cells, the nucleus divides before the cell itself divides. The process in which the nucleus divides is called mitosis. Before mitosis occurs, a cell’s DNA is replicated. This is necessary so that each daughter cell will have a complete copy of the genetic material from the parent cell. How is the replicated DNA sorted and separated so that each daughter cell gets a complete set of the genetic material? To understand how this happens, you need to know more chromosomes. 170 CHROMOSOMES Chromosomes are coiled structures made of DNA and proteins. Each chromosome is a single DNA molecule. Chromosomes are the form of the genetic material of a cell during cell division. During other phases of the cell cycle, DNA is not coiled into chromosomes. Instead, it exists as a grainy material called chromatin. In eukaryotic cells during the S phase of interphase, DNA wraps tightly around disc-shaped proteins called histones. The DNA wraps around each histone twice. Histones help chromosomes maintain their shape and aid in the process of tightly packing DNA. When DNA is wrapped around the histones it forms bead-like structures called nucleosomes. Long strings of nucleosomes form thick coils that pack together to make up closed chromatin. During the prophase step of mitosis the chromatin, which is very tightly packed, become chromosomes that are visible under a microscope. These structural changes can be seen in Figure 6.14 below. Figure 6.14 DNA’s transition into chromosome form. Chromosomes are simpler structures in prokaryotic organisms. The DNA of most prokaryotes consists of only one chromosome, which is attached to the inside of the cell membrane. Prokaryotic chromosomes consist of a circular DNA molecule (see Figure 6.15) compactly packed into the cell. Figure 6.15 Prokaryotic chromosomes consist of a single circular DNA molecule, while eukaryotic chromosomes are composed of tightly packed chromatin. Chromatids and the Centromere DNA condenses and coils into the familiar X-shaped form of a chromosome, shown in Figure 6.16, only after it has replicated. This X-shaped form of DNA has already replicated, each chromosome actually consists of two identical copies. The two copies are called sister chromatids. They are attached to one another at a region called the centromere. 171 Figure 6.16 After DNA replicates, it forms chromosomes like the one shown here. Chromosome Numbers Every species of living organism on Earth has its own characteristic number of chromosomes in each of its cells. For example, cats have 32 chromosomes in each of their cells and dogs have 78. Some species even have the same number of chromosomes in each of their cells. Potatoes, plums, and chimpanzees all have 48 chromosomes in each of their cells. Human cells normally have two sets of chromosomes, one set inherited from each parent. There are 23 chromosomes in each set, for a total of 46 chromosomes per cell. Each chromosome in one set is matched by a chromosome of the same type in the other set, so there are actually 23 pairs of chromosomes per cell. Each pair consists of chromosomes of the same size and shape that also contain the same genes. The chromosomes in a pair are known as homologous chromosomes. DISCOVER OF DNA STRUCTURE Today, it is commonly known that DNA is the genetic material. For a long time, scientists knew such molecules existed. They were aware that genetic information was contained within organic molecules. However, they didn’t know which type of molecules play this role. In fact, for many decades, scientists thought that proteins were the molecules that carry genetic information. In this section, you will learn how scientists discovered that DNA carries the code of life. Griffith Searches for the Genetic Material Many scientists contributed to the identification of DNA as the genetic material. In the 1920s, Frederick Griffith made an important discovery. He was studying two different strains of a bacterium, called R (rough) strain and S (smooth) strain. He injected the two strains into mice. The S strain killed (virulent) the mice, but the R strain did not (non-virulent) (see Figure 6.17). Griffith also injected mice with S-strain bacteria that had been killed by heat. As expected, the killed bacteria did not harm the mice. However, when the dead S-strain bacteria were mixed with live R-strain bacteria and injected, the mice died. Figure 6.17 Griffith’s Experimental Results. Griffith showed that a substance could be transferred to harmless bacteria and make them deadly. 172 Based on his observations, Griffith deduced that something in the killed S-strain was transferred to the previously harmless R-strain, making the R-strain deadly. He called this process transformation, as something was "transforming" the bacteria from one strain into another strain. What was that something? What type of substance could change the characteristics of the organism that received it? Avery’s Team Makes a Major Contribution In the early 1940s, a team of scientists led by Oswald Avery tried to answer the question raised by Griffith’s results. They inactivated various substances in the S-strain bacteria. They then killed the S- strain bacteria and mixed the remains with live R-strain bacteria. (Keep in mind, the R-strain bacteria usually did not harm the mice.) When they inactivated proteins, the R-strain was deadly to the injected mice. This ruled out proteins as the genetic material. Why? Even without the S-strain proteins, the R- strain was changed, or transformed, into the deadly strain. However, when the researchers inactivated DNA in the S-strain, the R-strain remained harmless. This led to the conclusion that DNA is the substance that controls the characteristics of organisms. In other words, DNA is the genetic material. Hershey and Chase Seal the Deal The conclusion that DNA is the genetic material was not widely accepted at first. It had to be confirmed by other research. In the 1950s, Alfred Hershey and Martha Chase did experiments with viruses and bacteria. Viruses are not cells. They are basically DNA inside a protein coat. To reproduce, a virus must insert its own genetic material into a cell (such as a bacterium). Then it uses the cell’s machinery to make more viruses. The researchers used different radioactive elements to label the DNA and proteins in viruses. This allowed them to identify which molecule the viruses inserted into bacteria. DNA was the molecule they identified. Their experimental procedure can be viewed in Figure 6.18. This confirmed that DNA is the genetic material. Figure 6.18 Hershey and Chase experiments using radioactive markers to trace the genetic material transferred from viruses to bacterium. Chargaff Writes the Rules Other important discoveries about DNA were made in the mid-1900s by Erwin Chargaff. He studied DNA from many different species. He was especially interested in the four different nitrogen bases of DNA: adenine (A), guanine (G), cytosine (C), and thymine (T) (see Figure 6.19). Chargaff found that concentrations of the four bases differed from one species to another. However, within each 173 species, the concentration of adenine was always about the same as the concentration of thymine. The same was true of the concentrations of guanine and cytosine. These observations came to be known as Chargaff’s rules; A pairs with T and G pairs with C. The significance of the rules would not be revealed until the structure of DNA was discovered. Figure 6.19 Nitrogen Bases in DNA. The DNA of all species has the same four nitrogen bases. The Double Helix After DNA was found to be the genetic material, scientists wanted to learn more about it. James Watson and Francis Crick are usually given credit for discovering that DNA has a double helix shape (April, 1953), like a spiral staircase (see Figure 6.20). The discovery was based on the prior work of Rosalind Franklin and other scientists, who had used X rays to learn more about DNA’s structure. Franklin and these other scientists have not always been given credit for their contributions, see Figure 6.21 of Rosalind Franklin’s x-ray diffraction images of the DNA molecule.